Happy Birth Day to Mei?

Tai Shan might have to move over. The National Zoo could be celebrating the birth of a second giant panda cub this summer.

Or not.

There are signs that Tai Shan's mother, Mei Xiang, is pregnant again, zoo officials said yesterday. They quickly cautioned that pandas can have false pregnancies that mimic real ones; Mei Xiang had such a pseudo pregnancy in 2004. As with Tai Shan, who was born the next year, the pregnancy watch could continue right to the last minute -- in July -- with no one knowing for certain.

The buzz began Wednesday when the mama panda's hormone levels spiked, two months after she went into heat and was artificially inseminated twice over two days with sperm from a male panda named Gao Gao. His sperm had been frozen and flown, first-class, from the San Diego Zoo.

But with pandas, there's no such thing as a reliable pregnancy test. Whether or not the insemination worked, Mei Xiang's hormone levels were expected to spike at some point. Then the waiting would begin.

"This increase in hormones tells us nothing more than we're at a certain point in the process -- whichever of the two processes we're talking about," said zoo spokesman John Gibbons. "Just like with Tai Shan, we might not know until the actual birth."

To say that the zoo's scientists are tracking developments would be an understatement. Hormone levels are being tested daily. There is also a possibility that the hormonal surge will disappear within a day or so -- meaning that it was simply a random spike, unrelated to a pregnancy false or true.

If they stay elevated, the truth likely will emerge in 30 to 50 days, Gibbons said. At that point, the hormone levels will drop, and a baby panda will be born, or the pregnancy will be proven false.

Modern technology offers one other tool: an ultrasound. A panda fetus would not show up on a scan quite this early, Gibbons said, but Mei Xiang will undergo an ultrasound exam each week, and close to her potential due date something could be visible. But there's a catch: When she was pregnant with Tai Shan, Mei Xiang refused to cooperate with the ultrasound.

Tai Shan, whose birth July 9, 2005, created international excitement, is also a product of artificial insemination. His father is Tian Tian, who was brought from China to Washington along with Mei Xiang in December 2000, under a 10-year, $10 million agreement.

The entire family -- hugely popular among zoo visitors -- is on loan from the government of China, as would be any future Mei Xiang offspring.

Tian Tian is still healthy. But this time, the zoo chose to use sperm from Gao Gao because he has a rarer genetic line that is especially valued in global breeding.